The present invention relates to an oscillator type accelerometer of the type wherein a mass is hingedly fixed to a support frame in a cantilever fashion, an oscillator is fixedly mounted on one side of the mass between its free end portion and the support frame at right angles to both the longitudinal direction of the mass and its turning axis, the free end portion of the mass is angularly displaced with respect to the support frame when acceleration is applied to the latter, and a change in the natural oscillation frequency of the oscillator by the angular displacement is detected to thereby detect the applied acceleration.
A description will be give first, with reference to FIGS. 1 and 2, of a prior art example on which the present invention is based. FIG. 1 schematically shows the overall structure of a conventional oscillator type accelerometer employing two oscillators. A support frame 11 is provided with a support portion 11c and leg portions 11a and 11b and is U-shaped as a whole. The support portion 11c has secured to the center thereof one end of a mass 12 through a thin hinge 13 so that the mass 12 extends in parallel to both leg portions 11a and 11b. In this example, the mass 12 has its one end coupled to the support portion 11c through the thin hinge 13 which spreads in a direction perpendicular to the plane containing the leg portions 11a, 11b and the support portion 11c of the support frame 11. Hence, the free end portion of the mass 12 can be displaced toward either of the leg portions 11a and 11b. Fixed at one end to the free end portion of the mass 12 are a pair of oscillators 14 and 15 which are secured at the other end to the leg portions 11a and 11b, respectively. As shown in FIG. 2, the oscillators 14 and 15 each have two parallel square-bar-shaped oscillatory portions 16 and 17 coupled to one another at both ends thereof, each having three electrodes 18 formed on each of its four surfaces lengthwise thereof. By applying an AC signal across the electrodes 18, the oscillatory portions 16 and 17 are caused to perform flexural oscillation in a direction perpendicular to their longitudinal direction in opposite phases to each other as depicted in FIG. 3. The oscillators 14 and 15 are usually quartz oscillators and they are excited at their natural oscillation frequency.
In FIG. 1, when an upward acceleration is applied to the support frame 11 as indicated by the arrow 19, the mass 12 is urged by inertia to move relative to the support frame 11 in a direction opposite to that indicated by the arrow 19, and hence is elastically displaced about the thin hinge 13. In this example the oscillator 14 receives a tensile stress and the oscillator 15 a compressive stress. As a result, the natural oscillation frequency of the oscillator 14 rises, whereas the natural oscillation frequency of the oscillator 15 lowers. The direction and magnitude of the applied acceleration can be detected by measuring the natural oscillation frequencies of the oscillators 14 and 15 and detecting the difference therebetween. The applied acceleration could be detected even with the use of only one of the oscillators 14 and 15, but when only one oscillator is employed, a bad influence is exerted on the performance of the accelerometer by a fluctuation in the natural oscillation frequency of the oscillator which is caused by an ambient temperature change in the case of no acceleration being applied. The use of two oscillator lessens such a bad influence.
The natural oscillation frequency of each oscillator used in the above-mentioned oscillator type accelerometer depends on the oscillation mode and configuration of the oscillator itself, and in the case of an oscillator of a flexural oscillation mode, its natural oscillation frequency is around 200 kHz at the highest. Since the oscillator type accelerometer detect the applied acceleration as a variation in the natural oscillation frequency of the oscillator, the resolution of the applied acceleration is dependent on the extent to which the frequency variation can be identified. In the case where the frequency variation is predetermined with respect to a fixed applied acceleration, it is necessary, for enhancement of the resolution, to improve the ability of identifying the frequency variation. In the oscillator type accelerometer utilizing the oscillators of the flexural oscillation mode, their natural oscillation frequencies are as low as 40 KHz or so and the frequency variation is also a maximum of only 8 KHz. In this instance, if the sampling time for measurement is assumed to be 1 sec, the resolution of acceleration is only 1/8000 of the maximum applied acceleration, and if a resolution in six figures is to be obtained, then it will be necessary that a carrier frequency mentioned below be higher than 1 MHz.
In the case where the variation in the natural oscillation frequency of each oscillator is low as mentioned above, it is customary in the prior art, for improving the frequency variation identifying ability and the resolution of the applied acceleration, to utilize a method which lengthens the sampling time for measurement or waveform-shapes the output waveform into a rectangular wave and then measures its pulse width or pulse interval. Needless to say, however, the method of increasing the sampling time is not preferable in that it involves the calculation of the mean value of the applied acceleration throughout the sampling time. On the other hand, the pulse width or pulse interval of the rectangular wave, into which the output waveform is shaped, is measured by counting high-frequency oscillation output pulses from an oscillation circuit of a frequency (a carrier frequency) sufficiently higher than the natural oscillation frequency of each oscillator. This method calls for very complex electric circuitry and the oscillation frequency stability of the oscillation circuit is related directly to the performance of the oscillator type accelerometer, and instability of the oscillation frequency often impairs the performance of the accelerometer.
Another prior art example of the oscillator type accelerometer is one that adopts air damping for producing a damping effect on the mass 12. In this case, air is sealed in the case of the accelerometer so that the damping effect is produced by the viscous friction of the air with the mass 12. When the oscillators 14 and 15 are of the flexural oscillation mode, they oscillate with relatively large amplitudes as shown in FIG. 3, and hence are subjected to the damping effect. Since the oscillation of the oscillators 14 and 15 is thus suppressed, their Q values decrease. If an oscillation circuit is formed using such oscillators as oscillation frequency determining elements, then stability of its oscillation frequency will lower. This leads to lowering of the performance of the oscillator type accelerometer, in particular, its resolution.